This disclosure relates to novel, stable aqueous pigment dispersions, the polymeric dispersants that produce the stable aqueous pigment dispersions, the process of making the pigment dispersions and the use thereof in ink jet inks. These dispersants enable a unique combination of ionic and steric stabilization. In water, they provide only ionic stabilization with a random ionic block, but with addition of ink vehicle components, these dispersants facilitate entropic repulsion and steric stabilization with an ink vehicle soluble block.
Aqueous dispersions of pigments are known in the art and have been used in various applications such as, for example, inks for printing (particularly ink jet printing); waterborne paints and other coating formulations for vehicles, buildings, road markings and the like; cosmetics; pharmaceutical preparations; etc. Because pigments are typically not soluble in an aqueous ink vehicle, it is often required to use dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the ink vehicle.
An application of the present disclosure relates to an ink (printing liquid) useful for writing utensils such as aqueous ball point pens, fountain pens and felt-tip pens; continuous and on-demand type inkjet printers of a thermal jet type, a piezo type and the like; and an inkjet printing method employing the ink.
Aqueous pigment dispersions generally are stabilized by either a non-ionic or ionic technique. When the non-ionic technique is used, a polymer having a non-ionic hydrophilic section that extends into the water medium is typically employed. The hydrophilic section provides entropic or steric stabilization that stabilizes the pigment particles in the aqueous ink vehicle. Polyvinyl alcohol, cellulosics, ethylene oxide modified phenols and ethylene oxide/propylene oxide polymers may be used for this purpose.
While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage in that the printed image is water sensitive. Thus, non-ionic content should be minimized to ensure durability.
In the ionic technique, the pigment particles are stabilized using the polymer of an ion containing monomer, such as neutralized acrylic, maleic or vinyl sulfonic acid. The polymer provides stabilization through a charged double layer mechanism whereby ionic repulsion hinders the particles from flocculation. Since the neutralizing component tends to evaporate after printing, the polymer then has reduced water solubility and the printed image is not water sensitive.
There continues to be a need for higher-quality and different property inks for inkjet ink applications. For instance, photographic and other highly colored printing requires improved inkjet inks. Although improvements in polymeric dispersants have significantly contributed to improved inkjet inks, the current dispersants still do not provide inks with requisite optical density and chroma needed for emerging ink jet applications.